358 
REVIEW—THE HORSE’S MOUTH. 
dwell for a few moments on that interesting part of the subject, 
their “ composition.” 
“ Three substances enter into the composition of the horse’s 
tooth :— 1 . crusta petrosa or hard crust, enamel, and ivory!' 
“ The CRUSTA Petrosa is the most externally situated, and 
when the tooth first appears in the mouth, it is entirely coated by 
this substance, having a somewhat dull and dark appearance, 
which is only lost when the hard crust is removed by attrition, and 
the enamel thereby exposed. In amount, the crusta petrosa is 
second to ihe ivory, being more in quantity than the enamel. In 
composition, it is characterized by containing a great amount of 
animal substance ; and, in structure, it is peculiar for exhibiting, 
under the microscope, numerous corpuscles or cells, from which 
pores or minute tubes appear to radiate. Within the alveolar 
cavity, the crusta petrosa, which around the fang becomes of con¬ 
siderable thickness, is of a yellowish white colour; hut where, in 
connexion with the crown of the tooth, it is exposed to the che¬ 
mical action of the food and air, it presents a darker aspect and 
looks like an accumulation of tartar, for which indeed it has been 
mistaken. It fills up the infundibula of the molars of the upper 
jaw, and lines those of the incisors, being pierced by all the vessels 
which nourish the teeth. If a tooth be subjected to the action of 
dilute hydrochloric acid, the enamel will be removed, and the ivory 
and crusta petrosa be rendered separate and distinct.” 
Both English and French veterinary writers have mistaken the 
crusta petrosa for tartar, not being aware of its existence inside 
as well as outside the tooth. The earliest intimation we had of 
its existence arose from the perusal, some years ago, of a little 
work by Rennie, intituled “ The Alphabet of Zoology” in which 
we found it stated—“ In those quadrupeds which grind their food, 
such as the ox and the sheep, the tooth is composed of three pecu¬ 
liar textures, bone, enamel, and cement ( crusta petrosa vel cortex 
osseus), as may be seen on cutting an elephant’s tooth, or that of a 
horse or ox, in the direction of the jaw, and polishing the sur¬ 
faces.” 
On the subject of what is understood the “ after-growth” of the 
teeth—a property which was assigned them, ab origine , by the 
German veterinary Professor, Pessina, Mr. Mayhew expresses 
doubt:— 
“ I admit the teeth of the horse do grow, and only doubt if the 
